1. Field of the Invention
The present invention relates to a developing device in which toner particles are supplied to a static latent image formed on a peripheral surface of a photoconductive drum in an electrophotographic manner, and an image forming apparatus provided with the developing device.
2. Description of the Related Art
A predetermined developing device is used in an image forming apparatus which is operable to perform an electrophotographic manner. In the developing device, toner particles as a developer are supplied to a static latent image which is formed on a peripheral surface of a photoconductive drum in accordance with an image data. A toner image formed on the peripheral surface of the photoconductive drum by supplied toner particles as mentioned above is transferred onto a sheet which is an image bearing material conveyed from a sheet storage section with rotation of the photoconductive drum about an axis. A fixing device is provided on an immediately downstream of the photoconductive drum to perform a fixing process onto the sheet. A fixing process is conducted on the sheet onto which the toner image is transferred to fix the toner image on the sheet by heat. Then, the sheet is discharged to an outside after the completion of the fixing process.
The developing device includes stirring means for stirring a developer which includes only toner particles in the case of the one-component system, or a mixture of toner particles and carrier particles in the case of the two-component system, a cylindrical developing sleeve for supplying developer particles being stirred by the stirring means to the peripheral surface of the photoconductive drum, and a casing accommodating these members. In the case of the one-component system, toner particles supplied from a predetermined toner cartridge into the casing are stirred without adding an agent. In the case of the two-component system, toner particles are mixed and stirred with carrier particles put by a predetermined amount. Then, toner particles are supplied onto the peripheral surface of the photoconductive drum uniformly by a peripheral surface of the developing sleeve rotating about an axis. A blade is provided closely above the peripheral surface of the developing sleeve arranged along the peripheral surface of the photoconductive drum at a predetermined space. The blade removes surplus toner particles from the peripheral surface of the developing sleeve to thereby prevent excessive supply of toner particles to the photoconductive drum.
Meanwhile, there is the problem that when toner particles conveyed with the rotation of the developing sleeve reach the blade and some of the toner particles stray from a middle portion of the blade to the both ends of the blade, and disadvantageously leak from the respective end portions of the developing sleeve.
In order to solve this disadvantage, Japanese Unexamined Patent Publication No. HEI 10-48946 discloses that a half-ring shaped magnet is attached to each of the opposite side walls of the housing in a state of facing the end portion of the developing sleeve, and the magnet has a plurality of sets of opposite magnetic poles. The magnets are respectively disposed in the state of facing the peripheral surface of the both end portions of the developing sleeve to thereby produce a magnetic flux from one pole to the other pole of each set of magnetic poles, and the magnetic fluxes run in a space between an inner surface of the half-ring shaped magnet and the peripheral surface of the developing sleeve. Consequently, straying toner particles are prevented from leaking by the magnetic fluxes.
However, in the developing device disclosed in Japanese Unexamined Patent Publication No. HEI 10-48946, the magnetic fluxes merely run end to end of each set of magnetic poles in the space between the developing sleeve and the magnet. It will be seen that the magnet cannot effectively prevent straying toner particles from leaking. In particular, in machines aimed at performing image forming processing at a high speed, the developing sleeve is rotated at a high speed. Accordingly, toner particles have a greater inertial force. Thus, it is hard to prevent the toner leakage only by the end-to-end magnetic fluxes caused by the sets of magnetic poles provided on the magnet.